The present invention is generally directed to a system and method for displaying surface information. More particularly, the present invention incorporates non-linear interpolation to remove certain image artifacts. The images of the surfaces displayed are typically contained within the interior regions of solid bodies which are examined by computed axial tomographic (CAT) x-ray systems or by nuclear magnetic resonance (NMR) imaging systems either of which is capable of generating three dimensional arrays of data representative of one or more physical properties at various locations within a three dimensional volume. The present invention is especially directed to a system and method for the display of medical images so as to obtain images and representations of internal bodily structures. The images generated in the practice of the present invention provide three dimensional data for examination by physicians, radiologists, and other medical practitioners. The present application has been filed concurrently with application Ser. No. 741,390.
In conventional x-ray systems, a two dimensional shadow image is created based upon the different x-ray absorption characteristics of bone and soft tissues. A great improvement on the conventional x-ray system as a diagnostic tool was provided by the computed axial tomographic systems which have been developed over the last ten years or so. These so-called CAT systems are x-ray based and initially were used to produce single two dimensional views depicting transverse slices of a body, object, or patient being investigated. Three dimensional information was thereafter gleaned from CAT scan data by generating data for a number of contiguous slices and using the inferential abilities of the radiologist to suggest a three dimensional representation for the various internal organs. In one embodiment of the present invention, shaded and contoured three dimensional images are generated from the three dimensional array of data generated by a sequence of such contiguous CAT scans. However, resolution in the z-direction, perpendicular to each slice, is usually lower than the resolution within a slice. This can produce undesirable artifacts which are significantly reduced in the present invention. Similarly, the newer NMR imaging technology is also capable of generating three dimensional arrays of data representing physical properties of interior bodily organs. Moreover, NMR systems have the capability to better discriminate between various tissue types, not just bone and soft tissue. NMR imaging systems are also capable of generating physiological data rather than just image data. However, whether NMR or CAT systems are employed, the data has been made available only as a sequence of slices and systems have not generally been available which provide true three dimensional images.
In the present invention, three dimensional data generated either by a CAT scanning system or by an NMR imaging system may be displayed and analyzed in a plurality of ways so as to produce on a display screen or other device a multitude of anatomical features which are selectable at the viewer's choice. In the system and method of the present invention, the data used to produce the three dimensional images is typically acquired once and then used and re-used to generate medical information and display images at the option of the viewer. The viewer is provided with the option of selecting one or more threshhold values which determine, for example, whether or not bone surfaces as opposed to brain surface tissue is to be displayed. The viewer or operator of the present system can also select the appropriate viewing angle and can, at will, selectively ignore segments of the data generated in order to provide cross sectional views through any desired plane. Moreover, the viewing angle is selectable and it is possible to generate a sequence of images and display them sequentially to provide the medical practitioner with interior views of solid surfaces in a truly three dimensional manner from any desired viewing angle with the further capability of being able to construct a view through any plane or slice. Again, it is pointed out that for many purposes, an almost infinite variety of meaningful images may be created from only a single set of NMR or CAT scan slice data arrays. Certainly though, if the objective of the medical investigation is the study of internal anatomic variations as a function of time, then it is meaningful to produce a sequence of three dimensional data arrays indexed by time. The system and method of the present invention provide the medical practitioner, and surgeons in particular, with the ability to plan detailed and complicated surgical procedures using totally non-invasive diagnostic methods. The images generated by the present invention can only be described as truly dramatic and show every evidence of being as great an improvement in the medical imaging arts as computed axial tomography and nuclear magnetic resonance imaging.
While the system and method of the present invention will undoubtedly find its greatest utilization in the analysis and display of tomographic x-ray and nuclear magnetic resonance imaging data, the system of the present invention is equally applicable to systems employing ultrasound, positron emission tomography, ECT (emission computed tomography) and MMI (multimodality imaging). Moreover, while the present invention is particularly applicable to the construction of medical images, it is also pointed out that the system and method of the present invention is applicable to the display of interior three dimensional surface structures for any system which is capable of generating three dimensional data arrays in which signal patterns are present which represent the value of at least one physical property associated with points in a solid body.
A particular advantage of the present invention is its ability to provide the medical practitioner with the ability to perform interactive functions with the machine in real time. Systems which do not permit interactive use suffer a significant disadvantage since a real time display methodology is required for optimal human interaction with the system, particularly in the case of a surgeon planning a difficult procedure. For example, in transplant surgery, it is often difficult to ascertain beforehand the precise shape or size of a body cavity which is to receive the implant. This is true whether or not the implant comprises human tissue or a mechanical device. It is therefore seen that it would be very important for a surgeon to be able to display the cavity in question on a screen in three dimensional form and be able to rotate it and section it at will, before any invasive procedure is undertaken. It is also important to such medical practitioners that the images generated are sharp and exhibit excellent contrast. The images generated should also depict surface texture wherever this is possible.
The display of three dimensional graphic images on a cathode ray tube (CRT) screen has principally been driven by the goals and directions of computer aided design (CAD) and computer aided manufacture (CAM). Systems have been developed for displaying solid bodies and for manipulating images in desirable fashions to create solid models for manufactured parts and for rotating and viewing these parts from a multiplicity of directions. In particular, CAD/CAM systems have been developed which accept so called wire-frame data. In a wire-frame display format, the display processor is provided with a sequence or list of three dimensional points representative of the end points of line segments. These line segments are joined to represent various surfaces. An advantage of these wire frame images is the ability to rapidly rotate the image about various axes to obtain different views. This is actually a computationally burdensome task which has only recently been satisfactorily solved through the utilization of high speed, large scale integrated circuit devices. A wire-frame image, even one which has been processed to eliminate hidden lines, may typically comprise a list of 50,000 vectors which is displayed on a screen, each "vector" being a (directed) line segment drawn between two points by a CRT form of display device. More sophisticated graphics processing systems not only accept wire-frame data, but also perform functions such as hidden line removal and continuous shading in various colors and/or in various shades of gray. In such systems, the viewing angle is selected by the operator. In systems displaying shaded images, the normal vector to the surface being displayed varies from point to point on the surface and the shading is determined from this normal vector and the viewing angle. Thus, it is seen that the information provided by the normal vector is very important in the shading which is applied to what is in actuality a two dimensional CRT screen image. It is the shading of this image (based originally on wire-frame data) that creates a very effective illusion of three dimensions. It is the providing of these three dimensional clues (shading) to the human visual system which is achieved particularly well in the system of the present invention. The shading data is generally produced from wire-frame data in various graphics processing systems which operate to convert the vector format of wireframe images to the so-called raster scan format. It is the raster scan format that is most familiar to the television viewer. In general, computer graphics display systems and display methodologies are divided into vector based systems which are particularly appropriate for line type drawings and raster format systems which produce images which are more closely related to that which is seen by the human eye.
Related work in the field of displaying three dimensional images has been carried out by Gabor Herman who has employed a method in which each adjacent volume element is analyzed and quantized to discrete zero and one values. Surface approximations are made only by considering cube faces and surface normal information can only be partially reconstructed because of the quantization step that is performed. The resulting method also is computationally long and produces low resolution images and appears be slower than the present method for interactive use at this time.
Meagher, working for Phoenix Data Systems, has employed a method of octree coding in which the three dimensional data array is subdivided into eight regions and each region is subdivided until individual volume elements are formed. Regions not containing surfaces are not subdivided. However, this method requires special purpose hardware, while the images are crisp, individual volume elements produce a quantized artifact that is not observed in smooth tissues such as bone. Furthermore, octree encoding methods are incompatible with polygonal surface descriptions which are used in most of the advanced graphics display systems that have been developed for CAD/CAM technology. Other methods for displaying three dimensional data are, for example, described in U.S. Pat. No. 4,475,104 issued Oct. 2, 1984 in the name of Tsu Y. Shen. This patent appears to disclose a three dimensional display system which incorporates a depth buffer to provide separate 3D information as part of the mechanism for generating appropriate shading values.
Accordingly, it is seen that it is an object of the present invention to provide a system and method for the display of three dimensional information.
It is a further object of the present invention to provide a display system for use in conjunction with CAT scanners, ultrasound devices, NMR imaging systems, and any and all other systems capable of generating three dimensional data representative of one or more physical properties within a body to be studied.
It is yet another object of the present invention to provide a graphics system for medical images which is capable of interactive use and yet at the same time produces high quality images providing textural, shading, and other visual clues to the user.
It is yet another object of the present invention to provide a three dimensional graphics display system which is compatible with current CAD/CAM systems.
Another object of the present invention includes the ability to generate and display three dimensional wire frame information.
Still another object of the present invention is to maximize the information contained in a three dimensional data array for the purpose of surface representation.
It is also an object of the present invention to provide a system and method which is readily fabricatable in specialized electronic hardware including integrated circuit chips and read only memory (ROM) devices.
It is yet another object of the present invention to provide medical practitioners with the ability to emulate surgical procedures graphically prior to undertaking invasive measures.
Additionally, it is an object of the present invention to provide a plurality of three dimensional surface views from a single set of collected data.
Lastly, but not limited hereto, it is an object of the present invention to provide a system and method for display of three dimensional images of internal surface structures in such a way that the specific viewing angle and cross sectional viewing plane may be selected by the user in an interactive manner.